The purpose of this extension is to provide a pointer to a Questionnaire that will provide an 'answer' to the question. Questionnaire does not support answers of type 'element' or most complex data types. The only complex data types it does support are Reference, Quantity, Coding and Attachment. We don't think there's a strong use-case for having Questionnaires to fill out the contents of these, thus the extension is scoped to being the target of the Reference. Note, this doesn't mean that you couldn't have a Questionnaire that would result in an Address or Patient.contact, only that such a Questionnaire couldn't be used here (because the answer to a question could never be an Address or a Patient.contact). We will evaluate our mapping guidance to allow for Questionnaires that map to complex elements that are not fullblown resources